The GM calls for an ability check when you attempt a task (other than an attack) that has a chance of failure. (77) For each check, the GM decides:

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• which ability is relevant to the task at hand;

• whether a skill is applicable; and

• the task's Difficulty Class (DC) or if the contest rules apply.

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Your ability check = d20 + relevant ability modifier + proficiency bonus (if any) + other bonus or penalty.

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• See reasons for having advantage or disadvantage. 

• Add your proficiency bonus if a skill applies and you are proficient in it; otherwise roll without the bonus.

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You succeed when your ability check total ≥ the DC. Otherwise you fail.